Don;t know where you sourced yer foam but there's 2 part liquid available that will produce quite a bit of pourable for one helluva lot less than $200/sheet.
In 2#, 3#, 4# and 8#/cu.ft. densities available for varying degrees of structural use . The 4# is recommended for boat decks and moderate loads while the 8# is useful for structural infill. Ya could buy enuff 2# to cast a block large enuff to damm'd near sculpt a boat for $200
I've used box-store urethane sheet for slab work. Never saw any that exceeded $22/sheet unless it was very much thicker than 1". I've molded over common construction foam; both pourable and sheet .
It works better to have a mold to build in; but outer lams are entirely possible; given that you need to undersize and work out to finished dimensions. Look fwd to lotsa fill and fairing
Might be my solution to rotten cored decks! Section out and remove rot. Clean and fill/inject. Pop section back on and clamp. Bingo! new foam core deck!